High frequency couplers



April 26, 1960 R. KYHL HIGH FREQUENCY COUPLERS 5 Shets-Sheet 2 Filed Nov. 14, 1955 April 26, 1960 R. L. KYHL 2,934,719

HIGH FREQUENCY COUPLERS Filed Nov. 14, 1955 3 Sheets-Sheet 5 1. 2,934,719- HIGH FREQUENCY COUPLERS:

Robert L. Kyhl, Schen'e'ctady,-N.Y., assign'or to General Electric Company, a corporation of New York: Application November 14, 1955, SerialNo; 546,537 12 Claims. (Cl. 333

My invention relates to broadband coaxial. coupler components and particularly to broadband coaxial directional couplers.

Most directional couplers. are inherently frequency sensitive with respect to coupling or directivity or both. The present invention relates particularly to an improved coupler which is essentially independent oflfrequency with respect to both directivity and' coupling. Inv its broader aspects, my invention is. not limited to directional couplers but may be embodied in other high. frequency coupling circuits, suchv as three-terminal pair networks for coupling a single concentric transmission line to a pair. of concentric transmission lines or a circuit. for coupling a pair of concentric transmission lines in whichztheinner. and outer conductors are inverted through a coupling region embodying my invention. Still further embodiments of my invention provide a transition coupling be tween a concentric type of line to a shielded parallel conductortransmission line and also a coupling between the conductors of a concentric transmission. line which are continuous through a coupling region and which are surrounded by a third conductor.

I have discovered that trans-mission line sections may be coupled over an extended region of length by providing a coupling section made up of a three-conductor. circuit including an outer or surrounding conductor with two spaced inner conductors. The coupling is. effective providingthe-cross section of the coupling section;.that is, the spacings of the conductors and/or their individual cross sections are varied gradually or slowly over the coupling length in a cyclic maner. This is equivalent to saying that the mutual capacities between the various pairs of conductors of the coupling region vary cycli'cl'y; Mathematical analysis indicates that couplers so designed are insensitive to frequency withthe amount of coupling depending entirely upon the sequence of mutual cap-acities of the three conductors involved over the length of the coupling region. In directional couplers, the directivity is likewise insensitive to frequency. 7

It is accordingly an important object of'my invention to provide a new and improved broadband high frequency coupling device for concentric-type circuits.

It is another object of my invenation to provide an improved transmission line coupler having athree-conductor coupling region over the length of which the mutual capacities of the conductors vary in a predetermined manner to provide coupling which is-ind'epend'ent of frequency.

- rounded. by an outer: conductor.

Figs; 4-9;.inclusive;. are sectional views-taken alongzthe lines 44,.55;.etc., of'Figs. 3a and 3b;

Fig. 10 isa plan view of a. modified: form of a four; terminal coupler embodying my invention and corresponding generally to Figs: 3a and 3b;

Figs. 11-15, inclusive, are sectional views takenv along the lines 111-1, 1212, etc., of. Fig. 10;.

Fig. 16 is a diagram on triangular coordinates representingv the mutualcapacities of. the three-conductor section of the coupler of Figs. 10-15, inclusive;

Fig. 17 shows the three mutual capacities illustrated in Fig. 16 plotted against axial length. of the coupling section;

Fig. 18 illustrates arthree-terminal concentricline cou-v pler embodying my invention;

Fig. 19 is asectional view taken alongthe line 19-42 of- Fig. 18;

Fig. 20 represents; a coupler'embodying my invention: in which the inner and outer conductors of the input and output transmission line-sections are inverted;

Fig; 21 illustrates. thea-pplication of my invention to a transmission line' coupler for connecting a concentric transmission line; with a shielded parallel transmission line section, and I Fig. 22. illustrates; the application ofv my invention. to an attenuating, coupling section for coupling two concentric transmission lines;

Referring now to-Fig. 1 of the drawing, there is shown a: schematic sectional view of a three-conductor transe missionline including two spaced inner conductors sur- These conductors are designated as conductors 1; 2 and 3. The characteristics of such' a line may be specified by'the threev mutual capacitiesperunit length;.C Cg ,.and' C 1, as: a function ofdistancealong: the three-conductor line. At other: points in the specification. and in. the claims the mutual. capacities per unit length have been referred to simply as mutual capacities and it will be understood that this. expression refers: to the capacities per unit length: I havefound that the coupling K indecibels between the transmission line including; conductors 1 and: 3: and the: transmission line including; conductors 2 and; 3 maybe: expressed, in: terms of" an integral over the length of'th'ecoupler, by. the: equation.

where A=Ci C +C C +C Cm and where 0. is a con It is still another object of my invention to provide an 1 stant angle depending upon the relation of the mutual capacities at the beginning and end of the coupling section. Where the. mutual capacities have the same values at the end of the coupling section as at the beginning, the value of' 0- is zero. It will be. noted that the value of the integral does not depend on the distance along the coupler but merely on the sequence of values of the" mutual capacities, C C23, and C Most important, the frequency does not appear anywhere in the expression and the interaction or coupling is not related to frequency. The expression depends for its validity on the taper of the conductors or, in other words, the rate of variation of the mutual capacities: between the conductors being slow compared to a wavelength of-the high frequency. Stated another way, thecoupling section transmission line directional coupler embodying my invention;

Figs. 3a and 3b together represent an enlarged plan view of the device of Fig. 1 with the upper half of the outer conductor removed to show the configuration of i ing'my invention. It also provides, for thoseprefering a' the conductors;

is several wavelengths long at the operating frequency. j i The mathematical derivation of the above equation not essential to the present invention but the equation and its derivation do provide a theoretical basis forthe. performance which I have observed in structures embody-j mathematical approach, a guide to thetype of variation of the three mutual capacities desired in the coupling section.

For a detailed description of a four-terminal directional coupler embodying my invention, reference may be had to Figs. 2-9, inclusive, of the accompanying drawing in which the concentric transmission lines are coupled together by a three-conductor coupling section having mutual capacities which vary with length of the coupling section in a cyclic manner to produce operation which is independent of frequency with respect to both coupling and directivity.

structurally, the device of Figs. 2-9 includes two con ductors 10 and 11 which are supported within a conducting body 12 formed by upper and lower mating parts 13 and 14. The mating parts are held together by suitable screws 15 extending through one part and threaded into the other. As will be more apparent from a consideration of the sectional views 4-9, inclusive, the mating parts 13 and 14 of the body 12 are machined out to provide the conducting walls 16 and 17 forming the outer conductors for a pair of concentric-type transmission lines which are completed by the inner conductors 10 and 11. In the coupling region, the walls 16 and 17 merge into a single wall of generally rectangular cross section 18, but having semicircular end portions. The outer conductor divides again near the right-hand end of the coupler structure to provide the outer conductor walls 19 and 20 of the two output concentric transmission line sections. As illustrated by the sectional view of Fig. 4 the input end of the coupler comprises two concentric transmission lines of identical cross section. In Fig. 5, the outer conductors have merged to provide a single outer conducting wall 18 surrounding the two inner conductors shown in cross section at 10a and 11a. As compared with conductors 10 and 11 at section 4-4, the conductor 10a is of larger cross section and the conductor 11a is smaller. In Fig. 6 the inner conductors are designated 10b and 11b and shown as flattened close together and beginning to approach the same size. In Fig. 7 taken at the center of the coupler, the conductors 10c and 11c are closely spaced, of equal cross section and similarly spaced with respect to the opposite halves of the surrounding conducting wall 18. Fig. 8 is the same as Fig. 6 except that the conductor 11d is now larger than conductor 10d and is displaced to the right. In a similar way, Fig. 9 is similar to Fig. except that conductor 11 is small. No view has been shown for the output end of the coupler corresponding to Fig. 4 for the input end but it will be understood that near the output end, the coupler returns to a symmetrical arrangement of two identical concentric transmission lines. It will be understood that the inner conductors are maintained in spaced relation within the outer conductors by means of suitable supports. Since dielectrics tend to cause reflections which detrimentally affect the operation of the coupler with respect to high frequencies, foamed plastic supports of polystyrene (not shown) which present a minimum volume of dielectric material may be used.

In the embodiment just described, it will be apparent that the mutual capacities between the conductors 10, 11 and 18, which will be designated C C and C corresponding to the numbers used in connection with Fig. l, progress through a cyclic variation in which the mutual capacity C is zero at a position corresponding to Fig. 4, reaches a maximum value at a position along the coupler corresponding to Fig. 7 and returns again to zero at the output end of the coupler. In a similar way, the capacities C and C between the inner conductors and 11, respectively, and the outer conductor 18 are equal and of a substantial value at the start of the position corresponding to Fig. 4. Capacity C between conductors 10 and 18 increases in the region of the coupler of Fig. 5, for example, and the capacity C between conductors 11 and 18 decreases in this region. Between Figs. 5 and 6,

the'C capacity reaches a maximum and starts to decrease while the C reaches a minimum. At the section shown in Fig. 7 corresponding to the maximum value of C12, C is equal in value to C but is decreasing while C is increasing. As shown in Figs. 8 and 9, these capacities continue to vary so that C passes through a maximum and C passes through a minimum. With the arrangement shown in which the structure at the output, end is the same as that at the input end, the mutual capacities return to the same final values as their initial values.

Before discussing in detail the characteristics of the specific embodiment of a coupler embodying my invention just described and particularly the considerations involved in determining the geometry of the cross sections of the coupling section, a second four-terminal network as illustrated in Figs. 10-14, inclusive, embodying my invention will be described. This coupler is in general very similar to that described in connection with Figs. 2-9 but the shape and positioning of the conductors with length of the coupling section are somewhat different. Referring now to Figs. 10-15 of the drawing, this embodiment of my invention includes an input end including inner conductors 20 and 21 of two concentric transmission lines which are completed by surrounding conducting surfaces 22 and 23 formed in mating conducting pieces 24 and 24a. In arnanner very similar to that described in connection with Figs. 3 and 4, the outer conducting surfaces merge into a single surface 25 which forms the third and surrounding conductor of the coupling section. The other two conductors 20 and 21 proceed through a progressive variation in cross section and spacing so that the mutual capacities between the three conductors 20, 21 and 25 throughout the coupling .section proceed through a cycle of variation such as that shown in Fig. 17, forexample. In Fig. 11 which is taken along the line 11-11 of Fig. 10, conductor 20 designated as 20a at this cross section is shown as having become very small and conductor 21a as having become quite large. The outer conducting surface 25a is essentially rectangular with semicircular ends. Between section lines 11 and 12, the conductors 20 and 21 become transposed, conductor 20 passing through an opening formed in conductor 21 at the region 26 as shown in Fig. 10. Thus, in Fig. 12, conductor 20b is shown on the right with 21b on the left and surrounding three sides of conductor 20b. The outer conducting surface 25b approaches a circular cross section as a convenient configuration. In Fig. 13 at the center of the coupling section the structure is completely symmetrical, the outer conducting surface 25c having become circular and inner conductors 20c and 21c being equal in cross section and symmetrically spaced about a vertical axis. The section at lines 14-14 shown in Fig. 14 is similar to Fig. 12 but reversed with respect to the inner conductors in that conductor 20d is larger and substantially surrounds conductor 21d on three sides. Likewise, Fig. 15 is similar to Fig. 11. It should be noted that conductor 21 passes through conductor 22 at the region 27 so that in Fig. 15, conductor 21 is again on the right. Proceeding to the right on the structure, it will be apparent that the coupling section diverges into two concentric output transmission lines at the outer end in which the conductors 20 and 21 are concentrically located within the conducting surfaces 22 and 23, respectively.

The capacity variation between the three conductors of the coupling section will be described in detail in connection with Figs. 16 and 17. The capacities between the inner conductors 20 and 21 have been designated by C and between the inner conductors and the outer conductors, respectively, as C and C This designation is used to simplify the description and corresponds to the generalized showing of the coupling section in Fig. 1.

In Fig. 16, the variation of the three capacities is shown on triangular coordinates with the start and finish ashame- J of the coupling section, that" is; the input and output ends being designated at the'center of the baseline. The capacity variation proceeds in a clockwise direction. This same variation" of capacity plotted against length of the coupling section is shown on rectangular coordinates in Fig. 17. As illustrated in Figures 16 and 17; in the region designated a where the input end includes two concentric transmission lines, the coupling between the two inner conductors is zero. At this region, the couplings between the inner and outer conductors are equal and of substantial value. As conductor 21 increases in size as shown in Fig. 11, the corresponding capacity C increases, while capacity C corresponding to the capacity between conductors 2t) and 25 decreases. This is illustrated at points b" in Figures 16 and 17. The capacity between conductor 29 and the outer conductor 25 is zero at the region 26' where conductor 20 passes through and is completely shielded by the conductor 21. This is illustrated at points in Figures 16 and 17. At the cross sectionof the conductors shown in Fig. 13, C and C are equal and of relatively small magnitude since the conductors are relatively small and each conductor shields the other from a substantial portion of the outer conductor. The capacity. between the inner conductors is at a maximum. These conditions are illustrated at points d in Figures 16 and 17. Proceeding along the coupling section, the capacities follow the same type of variation as illustrated at points e and f of Figures 16 and 17 and return to their initial values at the end of the coupling section, as illustrated again at points a at the right hand end' of Figure 17 and at the starting point of the circular plot in Figure 16, where the conductors diverge into two similar concentric transmission line sections.

In both of the arrangements described, the capacities proceed through a cyclic variation change, with each mutual capacity in the three-conductor coupling section progressing through maximum and minimum values. The maxirna occur in a predetermined sequence and the minima in that same sequence, although over a given length of the coupler, it will be understood that the minima are displaced. As shown in Fig. 17, the sequence is as follows: C minirnum, C -maximum, C minimum, C maximum, C --minimum, C maximum, and C minimum. Y

For the particular type of variation of cross section chosen for the modification shown in Figs. -16, the three mutual capacities all come to zero at some point in the coupling section. C and C become zero due to the piercing of one of the inner conductors by the other at'regions 26 and 27. This shows up in Fig. 16 as a circle plot tangential to the three base lines. In the modification shown in Figs. 2-9, the values of C and C3 do not come to Zero and the plot of capacities on a triangular diagram corresponding to Fig. 16 will be a circle tangential only to the base line C and C While a particular progressive variation of capacities has been chosen so that the-plot on the triangular coordinates is a circle, it will be readily understood that this type of progressive variation is not essential and that other variations, which enclose an area when plotted on triangular coordinates, may be employed. It is also not necessary that the coupling section terminate with the mutual capacities the same as those at the starting point and in such a case, the plot will not be a circle but rather will terminate atsome value of the capacities substantially difie'rentfrom those at the start. Other coupling sections illustrating this point will be described at a later point in the specification. As pointed out earlier, the constant angle 6 in Equation 1 is zero for those couplers in which the mutual capacities of the conductors are the same at the output and input ends of the coupler and takes on different values where this condition is not satisfied by the physical structure.

It is necessary for the successful carrying out of my same cyclic order.

inventionthafthe'variation in capacity be'slow" or grad ual with respect to a wavelength. That is, a mmplete' cycle of variation of the mutual capacities takes place over' a greater'distance than the wavelength of the frequency being transmitted. It is also not necessary that the coupling section employ only one cycle of capacitance variation and it may be repeated as many times as desired, provided the capacity variations remain in the For example, in Fig. 17, the mutual capacities reach their maximum values in the following order: C C and C As the capacity maxima repeat themselves in the coupling section, it is essential that they proceed in the same order, namely, C C and C31.

Actual physical embodiments of my invention built in accordance with Figs. 2-9 and Figs. 1015 operate as 20 db and 8 db couplers, respectively. The couplers shown in Figs. 2-9, inclusive, provided substantially constant coupling over a frequency range of 2,000 to 11,000megacycles and the embodiment of Figs. 1015, inclusive, gave a substantially constant coupling over a frequency range of 3,000 to 7,000 megacycles.

In the operation of the couplers thus far described, it

will be appreciated that power supplied to one transmission line, such as provided at the left-hand end of the coupler by conductors 10 and 16, is transmitted along that line to the output end of the coupler, conductors 10 and 20. The coupled power is transmitted to the righthand or output end of the coupler on the transmission line section provided by conductors 11 and'19. In other words, these would be considered forward Wave' couplers.

In Figs. 1822, I have shown other embodiments of my invention in which a coupling section is utilized to interconnect a concentric transmission lineat one end with various types of circuits at the other and with the coupling section in each case being made up of two inner conductors and an outer conductor, the mutual capacities of which vary in accordance with the teachings of my invention.

Referring now to Figs. 18 and 19, I have shown my invention applied to a coupler including a coupling section having inner conductors 30 and 31 and a surrounding outer conductor 32. At the left or input end, con-' ductors 30 and 31 provide the inner and outer conductors 30a and 31a, respectively, of a concentric transmission line. At the right-hand end of the drawing, the inner conductors diverge and provide inner conductors 30b and 31b of'concentric transmission lines which are cornpleted by conductors 32a and 32b which merge into the single outer conductor 32.

At the left-hand end of the coupling section, a tapered member 33 of lossy material is interposed between the; outer conductor 30b of the input transmission line and the outer conductor 32 to provide a broadband matched termination in a manner well known to the art. It will be .appreciated that the showing in Fig. 18 is somewhat schematic with respect to the shaping of' conductors 30 and 31 and that they may in fact have cross sections along the length of the coupling section similar to those shown in connection with the earlier described fourterminal embodiments of my invention. In the modifi-' cation of Figs. 18 and 19, the mutual capacities should satisfy Equation 1. In the case of the three-terminal network just described, the value of the angle 0 is 45.

In Fig. 20, I have illustrated a further embodiment of my invention in which the coupling section interconnects two concentric transmission lines having the inner and outer conductors inverted. That is, the inner conductor at one end'becomes the outer conductor at the outer end and vice versa: This arrangement is useful, particularly where it is desired to apply a direct current voltage to an inner conductor. As shown in Fig. 20, the coupling section includes two inner conductors 34 and 35 and an outer and surrounding conductor 36. The inner'cdndll'c'.

tors 34 and- 35 provide inner conductor 34a andouter conductor 35a, respectively, at the left-hand end and an outer conductor 34b and inner conductor 35b, respectively, at the other end. Taperedmembers, specifically frustro-conical members, of lossy' material 37 and 38 are interposed between the outer conductor 36 of the coupling section and the outer conductors 35a and 34b of the concentric transmission line sections. For this modification of my invention, the angle 8 of Equation 1 is zero degrees.

In Fig. 21, I have shown my invention applied to a coupler for connection between a concentric transmission line at one end and a shielded parallel conductor transmission line at the other. The coupler section again includes inner conductors 39 and 40 and a surrounding conductor 41. The inner conductors provide at one end, respectively, the outer conductor 39a and inner conductor 4th: of a concentric transmission line. At the opposite end, the inner conductors provide the parallel conductor 39b and 40b of a parallel-type transmission line. A frustro-conical member 42 of lossy material with respect to high frequency is interposed between outer conductor 41 and coupling section and the outer conductor 39a of the concentric transmission line. For the arrangement shown in Fig. 21, the angle of Equation 1 is 45.

In Fig. 22, I have shown my invention applied to an attenuator section for coupling two concentric transmission lines. The inner conductors of the coupling section 4-3 and 44 terminate at one end in inner and outer conductors 43a and 44a of a concentric transmission line and at the other end in conductors 43b and 44b, the inner and outer conductors, respectively, of a second concentric transmission line. The coupling section is surrounded by an outer conductor 45. Frustro-conical or other tapered members 46 and 4-7 of lossy material are interposed between the outer conductor 45 of the coupling section and the outer conductors 44a and 44b of the coupled concentric transmission lines to provide a matched termination as described before. It will be understood that the mutual capacities in this embodiment are designed to satisfy Equation 1 and that the value of the angle 0 in that equation is 0.

From the foregoing detailed description of a number of embodiments of my invention, together with a description of the design considerations involved, it will be apparent that I have provided a novel coupling section which is capable of many applications. It is particularly useful in connection with directional couplers where both the coupling and directivity are independent of frequency.

While I have shown and described particular embodiments of my invention, it will be apparent to those skilled in the art that modifications may be made without departing from my invention and I aim by the appended claims to cover any such modifications as fall within thetruespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A broadband high frequency transmission line directional coupler comprising a first pair of concentric transmission lines each including inner and outer conductors, a second pair of concentric transmission lines each including inner and outer conductors, a coupling section including two inner conductors forming respectively continuations of the inner conductors of said pairs and a surrounding conductor provided by the merging of the outer conductors of said pairs, the spacing, shape and size of the conductors of said coupling section having such variation with distance along said coupling section as to provide mutual capacities between the three conductors of said section which vary progressively with length of said coupling section to provide a maximum value and a minimum value of each mutual capacity at spaced points along said section and with the maximum values and minimum values of the mutual capacities occurring in the same cyclic order.

2. A broadband high frequency transmission line directional coupler comprising a first pair of concentric transmission lines each including inner and outer conductors, a second pair of concentric transmission lines each including inner and outer conductors, a coupling section including two inner conductors forming respectively continuations of the inner conductors of said pairs and a surrounding conductor provided by the merging of the outer conductors of said pairs, the spacing, shape and size of the conductors of said coupling section having such variation with distance along said coupling section as to provide mutual capacities between the three conductors of said section which vary progressively with length of said coupling section to provide a maximum value and a minimum value of each mutual capacity at spaced points along said section and with the maximum values and minimum values of each of the mutual capacities occurring in the same cyclic order and at pointsspaced along said section with respect to the maximum and minimum values of the remaining mutual capacities, and the sum of said mutual capacities being constant with length along said section.

3. A broadband high frequency transmission line directional coupler comprising a first pair of concentric transmission lines each including inner and outer conductors, a second pair of concentric transmission lines each including inner and outer conductors, a coupling section including two inner conductors forming respectively continuations of the inner conductors of said pairs and a surrounding conductor provided by the merging of the outer conductors of said pairs, the spacing, shape and size of the conductors of said coupling section having such variation with distance along said coupling section as to provide mutual capacities between the three conductors of said section which vary progressively with length of said coupling section to provide such mutual capacities that when plotted on triangular coordinates, the plot of the three mutual capacities with distance along the coupling section encloses an area within the triangle.

4. A broadband high frequency transmission line directional coupler comprising a first pair of concentric transmission lines each including inner and outer conductors, a second pair of concentric transmission lines each including inner and outer conductors, a coupling section including two inner conductors forming respectively continuations of the inner conductors of said pairs and a surrounding conductor provided by the merging of the outer conductors of said pairs, the spacing, shape and size of the conductors of said coupling section having such variation with distance along said coupling section as to provide mutual capacities between the three conductors of said section which vary progressively with length of said coupling section to provide such mutual capacities that when plotted on triangular coordinates, the plot of the three mutual capacities with distance along the coupling section encloses a circle within the triangle.

5 A broadband high frequency coupler for coupling transmission line sections, said coupler comprising a coupling section including a pair of spaced conductors 'and a surrounding conductor, the spacing, shape and size of said conductors having such variation with the length of said section as to provide mutual capacities between said conductors, each of which has a maximum and a minimum value spaced along the length of said coupler, th'e maximum value and minimum value of the mutual capacities occurring in the same cyclic order, said coupling section having a physical length which is longer than the wave length of the energy to be transmitted.

6. A broadband high frequency coupler for coupling transmission line sections, said coupler comprising a coupling section including a pair of spaced conductors and a surrounding conductor, said conductors having such size, shape and spacing along the length of said section as to provide mutual capacities therebetween which vary progressively along the length of said section with said plotted on triangular coordinates, enclosing an area.

. 7. A broadband high frequency coupler for coupling transmission line sections, said coupler comprising a coupling section including a pair of spaced conductors and a surrounding conductor, said conductors having such size, shape and spacing along the length'of said section as to provide mutual capacities therebetween varying progressively along the length of said section with said mutual capacities each having a maximum and minimum value along the length of said section and with the maximum and minimum values of mutual capacity between each pair of conductors occurring in the same cyclic order and at spaced points along said section with respect to the maximum and values of each of the remaining mutual capacities, the variation of said mutual capacities with length of said section being'such as to provide a circular plot on triangular coordinates.

8. A broadband high frequency transmission line coupler comprising a coupling section including two inner conductors and a surrounding conductor, the spacing, shape and size of said conductors having such variation with distance along said coupling section as to provide mutual capacities between the three conductors of said section which vary progressively with length along said coupling section and with each capacity having a maximum value and a minimum value at spaced points with the maximum and minimum values of each of the mutual capacities occurring in the same cyclic order, said inner conductors diverging at one end of said coupling section to provide respectively the inner conductors of two coaxial transmission line sections, said transmission line sections including outer conductors which merge to form the outer conductor of said coupling section, and said inner conductors extending from the opposite end of said coupling section to provide the inner and outer conductors of a third coaxial transmission line section.

9. A broadband high frequency transmission line coupler comprising a coupling section including'two inner conductors and a surrounding conductor, the spacing, shape and size of said conductors having such variation with distance along said coupling section as to provide mutual capacities between the three conductors of said section which vary with length along said coupling section and with each capacity having a maximum value and a minimum value at'spaced points with the maximum and minimum values of each of the capacities occurring in the same cyclic order, the variation of said mutual capacities, with length of said coupling section when plotted on triangular coordinates, enclosing an area, said inner conductors diverging at one end of said coupling section to provide respectively the inner conductors of two coaxial transmission line sections, said transmission line sections including outer conductors which merge to form the outer conductor of said coupling section, and said inner conductors extending from the opposite end of said coupling section to provide the inner and outer conductors of a third coaxial transmission line section.

10. A broadband high frequency transmission line coupler comprising a first concentric transmission line including inner and outer conductors, a second concentric transmission line including inner and outer conductors, a coupling section interposed between said tranmission lines and including two spaced inner conductors and a surrounding conductor spaced from both of said inner conductors, one of the inner conductors of said coupling section forming at one end an extension of the inner conductor of said first transmission line and at the other end a continuation of the inner conductor of said second transmission line, and the other inner conductor of said coupling section providing at one end a continuation of the outer conductor of said first transmission line and at the other end a continuation of the outer conductor of said second concentric transmission line, the spacing, shape and size of the conductors of said coupling section having such variation with distance along said coupling section as to provide mutual capacities between the three conductors of said section which vary progressively along the length of said coupling section with each mutual capacity having a maximum value and a minimum value at points spaced along said section with respect to the maximum and minimum values of each of the remaining mutual capacities and with said maxima and said minima occurring in the same cyclic order.

11. A broadband high frequency transmission line coupler comprising a first concentric transmission line including inner and outer conductors, a second concentric transmission line including inner and outer conductors, a coupling section interposed between said transmission lines and including two spaced innei' conductors and a surrounding conductor spaced from both of said inner conductors, one of the inner conductors of said coupling section forming at one end an extension of the inner conductor of said first transmission line and at the other end a continuation of the outer conductor of said second transmission line and the other inner conductor providing at opposite ends thereof continuations of the remaining conductors of said first and second concentric transmission lines, the spacing,

shape and size of the conductors of said coupling section having such variation with distance along said coupling section as to provide mutual capacities between the'three conductors of said section which vary progressively with length along said coupling section and with each mutual capacity having a maximum value and a minimum value at points spaced along said section with respect to the maximum and minimum values of each of the remaining mutual capacities.

12. A broadband high frequency transmission line coupler comprising a first concentric transmission line including inner and outer conductors, a second transmission line including two spaced inner conductors and a surrounding shielding conductor, acoupling section interposed between said transmission lines and including two spaced inner conductors and a surrounding conductor spaced from both of said inner conductors, said surroundingconductor forming an extension of the surrounding shielding conductor of said second transmission line, one of the inner conductors of said coupling section forming at one end an extension of the inner conductor of said first transmission line and at the other end a continuation of one of the two spaced inner conductors of said second transmission line and the other inner conductor of said coupling section providing at one end a continuation of the remaining inner conductor of said second transmission lineand at the other end a surrounding conductor for said concentric transmission line, the spacing, shape and size of the conductors of said cou pling section having such variation with distance along said coupling section as to provide mutual capacities References Cited in the file of this patent UNITED STATES PATENTS 2,486,818 Bowman Nov. 1, 1949 2,575,571 Wheeler Nov. 20, 1951 2,606,974 Wheeler Aug. 12, 1952 2,679,632 Bellows May 25, 1954 

